This application is based on application Ser. No. 182784/1997 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the invention
The present invention relates generally to an image forming apparatus such as a copying machine or a printer for developing an electrostatic latent image formed on an image carrying member by a developing device and a method of controlling the same, and more particularly, to an image forming apparatus so adapted as to make tone reproduction by a variable area toning method and a method of controlling the same.
2. Description of the Related Art
Conventionally known as an image forming apparatus such as a copying machine or a printer is an image forming apparatus utilizing, in reproducing the tone of a formed image, a tone production method by density of each element (hereinafter "variable area toning method") for forming a pixel having a plurality of dots and supplying a developer to one or more dots at a predetermined position in the pixel to reproduce a tone.
In the image forming apparatus utilizing such a variable area toning method, developing conditions are set such that the image density of a formed image (an output image density) increases under predetermined conditions as the ratio of dots, to which a developer is supplied, in a pixel (a dot area index) increases, as indicated by a solid line in FIG. 1.
In the image forming apparatus utilizing such a variable area toning method, however, tone characteristics change depending on changes in its operating environment. For example, when the image forming apparatus is used under a low temperature and low humidity environment or under a high temperature or high humidity environment, the change in the output image density with the increase in the dot area index differs from that indicated by the above-mentioned solid line, as shown in FIG. 1. That is, the output image density is lower than a desired density under the low temperature and low humidity environment, while being higher than the desired density under the high temperature and high humidity environment. In either case, when an image having a dot area index of approximately 50% is formed, the output image density greatly changes.
The higher the resolution of the image is, the more sensitive the change in the output image density based on the changes in the operating environment appears.
Therefore, the applicant of the present invention has disclosed a method of adjusting, in the case of a dot area index of approximately 50% at which an output image density generally changes based on changes in operating environment, developing conditions using a detection pattern having a high resolution which is greatly affected by the changes in the environment and a reference pattern having a low resolution which is hardly affected by the changes in the environment such that the density of an output image formed using the detection pattern is approximately the same as the density of an output image formed using the reference pattern, and preventing tone characteristics from changing depending on the changes in the operating environment, in the previous U.S. patent application Ser. No. 08/534,252, now U.S. Pat. No. 5,835,235.
Even when the tone characteristics are thus prevented from changing depending on the changes in the operating environment, there occurs a case where the output image density does not increase under predetermined conditions with the increase in the dot area index, so that good tone characteristics are not obtained.
For example, in a developing device for conveying a developer to a developing area opposite to an image carrying member by a developer carrying member and developing an electrostatic latent image formed in the image carrying member, consider a case where the developer carrying member and the image carrying member in the developing device are provided so as to be opposite to each other with predetermined spacing (a developing gap) interposed therebetween. In this case, when the developing gap varies, a point of inflection occurs in the change in output image density with the change in dot area index. That is, when the developing gap decreases, the output image density is lower than a predetermined value at a low dot area index, while being higher than a predetermined value at a high dot area index, as shown in FIG. 2. Conversely, when the developing gap increases, the output image density is higher than a predetermined value at a low dot area index, while being lower than a predetermined value at a high dot area index.
In any case, the output image density at a dot area index of approximately 50% is a value almost corresponding to a predetermined value. In the method disclosed in the previous application, therefore, the changes in the tone characteristics cannot be suitably corrected, so that good tone reproduction cannot be accomplished.
Consider a case where the developer carrying member is provided so as to be brought into contact with the image carrying member in the above-mentioned developing device. In this case, as the beam diameter of a laser beam used for forming the electrostatic latent image in the image carrying member changes, the tone characteristics in the image forming apparatus change. For example, even if such control is carried out that the output image density increases under predetermined conditions as the dot area index increases in a case where image formation is carried out using a laser beam having a beam diameter of 50 .mu.m, a point of inflection occurs in the change in output image density with the change in dot area index when the beam diameter of the laser beam becomes 65 .mu.m. That is, although the output image density at a dot area index of 50% becomes a value almost corresponding to a predetermined value, the output image density is lower than a predetermined value at a low dot area index, while being higher than a predetermined value at a high dot area index, as shown in FIG. 3.
Also in this case, therefore, the changes in the tone characteristics cannot be suitably corrected, so that good tone reproduction cannot be accomplished in the method disclosed in the previous application, as in the above-mentioned case.